1. Field of the Invention
The present invention relates to a photosensor, and an embodiment of the invention disclosed herein relates to a photosensor utilizing an oxide semiconductor and a semiconductor device including the photosensor. Further, an embodiment of the present invention relates to a light measurement method using the photosensor.
2. Description of the Related Art
A number of detectors used for detecting an electromagnetic wave are known, and for example, detectors having sensitivity to light from the ultraviolet range to the infrared range are collectively referred to as photosensors. Among them, a photosensor having sensitivity to light in a visible light range from a wavelength of 400 nm to a wavelength of 700 nm is particularly referred to as a visible light sensor, and many visible light sensors are used for devices which need luminance adjustment, on/off control, or the like depending on the usage environment.
Some display devices detect ambient brightness of a display portion to adjust the luminance of the display. This is because wasted electric power of the display device can be reduced by increasing visibility through detecting ambient brightness with the use of a photosensor and performing display with an appropriate luminance. For example, as examples of the display device including a photosensor for adjusting the luminance, mobile phones, computers, and the like can be given. Further, as well as the ambient brightness of the display portion, luminance of the backlight of a display device, in particular, a liquid crystal display device is also detected by a photosensor to adjust the luminance of a display screen.
A photosensor, for example, includes a photoelectric conversion element such as a photodiode in a light sensing portion and can detect illuminance based on the amount of current which flows to the photoelectric conversion element. In Patent Document 1, a charge accumulation type photosensor is disclosed. This photosensor measures illuminance by utilizing such a property that the amount of electric charge flowing through a photodiode varies in accordance with the illuminance. More specifically, the illuminance is measured in the following manner: after electric charge is accumulated in a condenser (capacitor), a change in potential caused by discharge of electricity by a constant current circuit (a constant current power supply) is detected by a comparator, the time needed for the change in potential is converted to a digital signal by a counter circuit and a latch circuit, and then the digital signal is output.
As a photosensor, a sensor in which characteristics are changed by light can be utilized. For example, it is well known that conductivity of an oxide semiconductor changes by reception of light, and this fact is described in Patent Document 2 and Patent Document 3. In these documents, a technique for controlling the threshold of a transistor including an oxide semiconductor is disclosed. Further, Patent Document 4 discloses an attempt to use an oxide semiconductor in a photosensor or a memory.